When American photographer Alec Soth landed at Heathrow earlier this year, he was looking forward to two things: working on a commission in Brighton for the city's upcoming photo biennial, and spending time with his young family, who had come along for the ride.

A customs official had other ideas. "He told me I had no work visa," says Soth, 40. "After threatening to put us back on the plane, he finally allowed us to stay, but said if I was caught taking photographs I could face up to two years in prison."

So Soth – best known for his off-beat, hauntingly banal images of modern America – was forced to think creatively. His seven-year-old daughter Carmen had always liked playing with his digital camera. "I'll sometimes give it to her and let her fire off 100 pictures and see what she comes up with," he says. "So I thought it would be interesting to see what she could do in Brighton."

Father and daughter strolled around the town for a few hours each day. "For the first couple of days, I didn't say much, I just watched her point at this and that," says Soth. Carmen photographed rubbish on the beach, other pedestrians, postcards on racks and the ground, often with her own sparkly red shoes in shot. But after a while, she ran out of steam. So she and Soth decided on certain subjects to look for, among them pushchairs and balloons, and the project started to take shape.

"I liked photographing dogs best of all," says Carmen. "I preferred garbage to people, but Dad said I should photograph them. Sometimes it was hard as they were walking quite fast."

In one picture, a girl with a bike tucks into a sandwich; in another, a woman in a mac walks straight at Carmen, oblivious to her. Some photographs are a little dark, some have too much flash, but they are fresh and uncontrived – and all taken from her distinctive, four-foot-tall perspective. "The hardest part was taking pictures in the rain. It rained every day," says Carmen. "And it got a bit tiring having the camera round my neck."

"It was incredible watching her work," says Soth. "She called portraits 'walk-bys', and most people had no idea she was taking their photograph. She was completely non-threatening." More interesting for Soth was Carmen's lack of photographic preconceptions. "Kids aren't drowning in cliches. They think it's just as interesting to shoot rubbish on a beach as a sunset, or a velvet rope rather than the museum behind it," he says. "She has a freewheeling way of framing. And by photographing anything she saw, she was completely democratic. My head is filled with nuanced cliches – I might subconsciously be influenced by William Egglestone – but she doesn't have that framework. Of course, as a professional, you can't go back to a time when you had none, but you can try. Myself, I long for that quality again."

This sort of naive perspective, either intentional or otherwise, is becoming increasingly prevalent in photography, says Soth. "We're highly visually literate right now, so I think people are responding to things that feel real and uncrafted. Take Wolfgang Tillmans – he obviously knows everything there is to know about photography, but he manages to look like he's not trying.

"Working with Carmen reminded me that the greatest photography is vernacular. Sometimes, not being professional can be an asset: look at the impact the pictures at Abu Ghraib had. They're some of the most important photojournalistic images of recent time."

"It's an intriguing collaboration," says photographer Martin Parr, guest curator of the Brighton Photo Biennial, where Carmen's work will be shown. "You can see their two strands of thought merging. As a photographer, you become more knowing, but Carmen's obviously not at that stage, which is a blessed relief. As you get older, you look back at that time with awe."

Carmen's pictures show that anyone can be a photographer, Parr believes. "It's the most democratic art form, but it's painfully difficult to be good at it."

Soth is best known for his large-scale American projects Sleeping by the Mississippi and Niagara, which took him across central America, from Iowa and Illinois to Missouri and Kentucky. Featuring images from portraits to bleak landscapes, they're in the grand tradition of road photography established by the likes of Walker Evans and Stephen Shore.

How did Parr react when Soth, one of the stars of the biennial, told him he wouldn't be showing his own work?

"I was worried it might scupper the project, but his solution was brilliant," says Parr. "I'm sure there were nerves," adds Soth. "If I heard about a project by a photographer's seven-year-old daughter, my reaction would be 'ugh'." Finally, he had to edit Carmen's 2,000 photographs into a cohesive collection. "I took the theme of springtime renewal – bunnies, flowers, pink things and so on, without being too heavy-handed," he says.

Are her photographs any good? "Yes, I think they could stand alongside any other professional work," says Soth, proudly. "I showed them to some students without telling them who'd taken them, and they were impressed. One observed that 'they were from a uniformly low perspective', which made me smile."

As for Carmen, how does she feel about a room full of strangers looking at her work? "If I don't think about older kids seeing them, then I'm OK," she says. "Teenagers are a little scary."